Carbohydrate Metabolism Practice Questions

Q: Which of the following is an aldose sugar?

Glyceraldehyde. ***Glyceraldehyde*** - **Glyceraldehyde** is the simplest **aldose**, a monosaccharide with an **aldehyde group** (CHO) at one end of its carbon chain. - Its chemical structure is a three-carbon chain with the aldehyde group on the first carbon, making it an **aldo sugar**. *Ribulose* - **Ribulose** is a **ketose**, specifically a **ketopentose**, meaning it is a five-carbon sugar with a **ketone group** (C=O) in its structure. - The ketone group in ribulose is typically located on the second carbon, distinguishing it from aldoses. *Fructose* - **Fructose** is another example of a **ketose**, specifically a **ketohexose**, as it is a six-carbon sugar containing a **ketone group**. - Its ketone group is usually found on the second carbon atom, which differentiates ketoses from aldoses structurally. *None of the options* - This option is incorrect because **glyceraldehyde** is indeed an aldose sugar, fitting the definition of a monosaccharide with an aldehyde functional group. - As **glyceraldehyde** is correctly identified as an aldose, this choice would contradict the chemical classification of sugars.

Q: In the oxidative phase of pentose phosphate pathway, NADPH is produced in?

Cytosol. ***Cytosol*** - The **oxidative phase** of the **pentose phosphate pathway (PPP)**, which produces **NADPH**, occurs exclusively in the **cytosol**. - Two key enzymes generate NADPH: **glucose-6-phosphate dehydrogenase (G6PD)** and **6-phosphogluconate dehydrogenase**. - **NADPH** is crucial for **reductive biosynthesis** (e.g., fatty acid synthesis, cholesterol synthesis) and for maintaining **redox balance** (e.g., reducing glutathione to protect against oxidative stress). *Mitochondria* - While mitochondria are central to **oxidative phosphorylation** and the **Krebs cycle**, they primarily produce **NADH** and **FADH2** for ATP generation. - The pentose phosphate pathway does not occur in mitochondria. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) and are not involved in metabolic pathways or NADPH production. - They are cellular machinery for translation, not metabolic compartments. *Peroxisomes* - **Peroxisomes** are involved in **fatty acid β-oxidation** and **detoxification** of hydrogen peroxide. - While peroxisomes have some oxidative enzymes, they are not the site of the pentose phosphate pathway or its NADPH production.

Q: Which of the following enzymes participates exclusively in glycolysis?

Pyruvate kinase. ***Pyruvate kinase*** - This enzyme catalyzes the **final step of glycolysis**, irreversibly converting **phosphoenolpyruvate (PEP)** to pyruvate, producing ATP. - **Exclusively participates in glycolysis** - it has no role in any other metabolic pathway, making it the most definitive answer. - All tissue-specific isoforms (M1, M2, L, R) perform the same glycolysis-exclusive function. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** catalyzes the committed step of glycolysis (Fructose-6-P → Fructose-1,6-BP) and is technically exclusive to the glycolytic pathway. - However, when the question refers to "phosphofructokinase" generically, it could include **PFK-2**, which produces fructose-2,6-bisphosphate (a regulatory molecule, not a glycolytic intermediate) and is part of the regulatory mechanism rather than the pathway itself. - **Pyruvate kinase is more unambiguously exclusive** to glycolysis as a metabolic enzyme. *Hexokinase* - While essential for the initial step of glycolysis, **hexokinase** phosphorylates multiple hexoses (glucose, mannose, fructose) and its product (G6P) can enter **multiple pathways**: glycolysis, pentose phosphate pathway, or glycogen synthesis. - **Not exclusive to glycolysis** - it serves as a branch point enzyme. *Glucose-6-phosphate dehydrogenase* - This enzyme is the rate-limiting step of the **pentose phosphate pathway (PPP)**, not glycolysis. - It catalyzes the oxidation of G6P to produce **NADPH** and ribose-5-phosphate for nucleotide synthesis, thereby diverting glucose-6-phosphate **away from glycolysis**.

Q: Which of the following is not a step in gluconeogenesis?

Conversion of pyruvate to acetyl-CoA. ***Conversion of pyruvate to acetyl-CoA*** - This step is a key irreversible reaction catalyzed by the **pyruvate dehydrogenase complex** that commits pyruvate to oxidative metabolism via the **Krebs cycle** or to fatty acid synthesis. - It is **not a part of gluconeogenesis**, as acetyl-CoA cannot be converted back to pyruvate or glucose in mammals. - This reaction is irreversible and represents a point of no return for glucose synthesis. *Conversion of glucose-6-phosphate to glucose* - This is the **final step in gluconeogenesis**, catalyzed by **glucose-6-phosphatase** in the liver and kidney. - This enzyme allows free glucose to be released into the bloodstream. - It is an essential gluconeogenic step that bypasses the irreversible hexokinase/glucokinase reaction of glycolysis. *Conversion of oxaloacetate to phosphoenolpyruvate* - This is a **key bypass step in gluconeogenesis** that overcomes the irreversible pyruvate kinase reaction in glycolysis. - It is catalyzed by **phosphoenolpyruvate carboxykinase (PEPCK)** and requires GTP. - This is crucial for synthesizing glucose from non-carbohydrate precursors like amino acids and lactate. *Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate* - This is an important **bypass step in gluconeogenesis**, catalyzed by **fructose-1,6-bisphosphatase**. - This irreversible reaction bypasses the phosphofructokinase-1 step of glycolysis. - It is one of the three key regulatory steps unique to gluconeogenesis.

Q: Which of the following enzymes do not catalyze an irreversible step in glycolysis?

Phosphoglycerate kinase. ***Phosphoglycerate kinase*** - This enzyme catalyzes the conversion of **1,3-bisphosphoglycerate** to **3-phosphoglycerate**, generating ATP. - This reaction is considered reversible because the free energy change is close to zero under physiological conditions, allowing for both forward (glycolysis) and reverse (gluconeogenesis) flux. *Hexokinase* - This enzyme catalyzes the **irreversible phosphorylation** of glucose to glucose-6-phosphate, trapping glucose within the cell. - It is one of the key regulatory enzymes in glycolysis, and its irreversibility ensures that glucose uptake and phosphorylation proceed in one direction. *Pyruvate kinase* - This enzyme catalyzes the final, **irreversible step** of glycolysis, converting phosphoenolpyruvate to pyruvate and generating ATP. - This reaction is a major control point for glycolysis due to its large negative free energy change. *Phosphofructokinase* - This enzyme catalyzes the **irreversible phosphorylation** of fructose-6-phosphate to fructose-1,6-bisphosphate. - It is considered the **rate-limiting step** and a primary control point in glycolysis, making it highly regulated and unidirectional.

Q: Which enzyme converts glucose to sorbitol?

Aldose reductase. ***Aldose reductase*** - This enzyme is crucial in the **polyol pathway**, reducing **glucose to sorbitol** by using **NADPH** as a cofactor. - In conditions of high glucose (e.g., uncontrolled diabetes), increased activity of **aldose reductase** leads to sorbitol accumulation, contributing to **osmotic damage** in certain tissues like the lens, nerves, and kidneys. *Sorbitol dehydrogenase* - This enzyme is responsible for the subsequent step in the polyol pathway, **oxidizing sorbitol to fructose** using NAD+ as a cofactor. - While related to sorbitol metabolism, it does not convert glucose to sorbitol; instead, it metabolizes sorbitol further. *Aldolase B* - This enzyme is involved in **fructose metabolism**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - It plays no direct role in the conversion of glucose to sorbitol. *Glucose-6-phosphate dehydrogenase* - This is the rate-limiting enzyme of the **pentose phosphate pathway**, catalyzing the oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone. - While it also uses NADPH (producing it rather than consuming it), it is not involved in the polyol pathway or sorbitol synthesis.

Q: Which of the following dietary fibers is most characteristically insoluble in water?

Lignin. ***Lignin*** - **Lignin** is a complex polymer found in plant cell walls, known for its **extreme insolubility** in water. - It provides structural rigidity to plants and is a non-carbohydrate component of **dietary fiber**. *Pectin* - **Pectin** is a type of soluble dietary fiber that forms a **gel-like substance** when mixed with water. - It is often used as a gelling agent in foods and is found in fruits like apples and citrus. *Hemicellulose* - **Hemicellulose** is a diverse group of polysaccharides; some forms are **soluble**, while others are **insoluble**, but it's generally more soluble than lignin. - Its solubility depends on its specific structure and sugar composition. *Cellulose* - **Cellulose** is an insoluble fiber, but it can absorb water and swell, contributing to **bulk in stool**. - While largely insoluble, **lignin** is considered the most characteristically insoluble fiber due to its highly cross-linked and rigid structure, which resists hydration even more effectively than cellulose.

Q: When the insulin to glucagon ratio decreases, which enzyme is primarily active?

Glycogen phosphorylase. ***Glycogen phosphorylase*** - A decrease in the **insulin-to-glucagon ratio** indicates a **low blood glucose** state, signaling the need for glucose production. - **Glycogen phosphorylase** is the key enzyme in **glycogenolysis**, which breaks down stored glycogen into glucose-1-phosphate, thereby elevating blood glucose. - This is the **primary and fastest response** to decreased insulin/glucagon ratio. *Fructose-1,6-bisphosphatase* - This enzyme is crucial for **gluconeogenesis**, specifically catalyzing the dephosphorylation of **fructose-1,6-bisphosphate** to **fructose-6-phosphate**. - While active during low insulin/high glucagon states, it is involved in synthesizing glucose, not directly breaking down stored glycogen as quickly as glycogen phosphorylase. *Pyruvate carboxylase* - This enzyme is the first committed step in **gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria. - Although active in response to a low insulin-to-glucagon ratio, its role is in synthesizing glucose from non-carbohydrate precursors, which is a slower process than immediate glycogen breakdown. *Glucose-6-phosphatase* - This enzyme is found primarily in the **liver and kidneys** and is responsible for dephosphorylating **glucose-6-phosphate** to free glucose, allowing it to exit the cell into the bloodstream. - While essential for the release of glucose from both gluconeogenesis and glycogenolysis, it acts at a later stage to make glucose available rather than initiating the breakdown of glycogen itself.

Q: An adolescent male patient presents to you with exercise intolerance. He gives a history of developing cramps on exertion. Which of the following enzyme deficiencies could be the cause?

Myophosphorylase. ***Myophosphorylase*** - A deficiency in **myophosphorylase** (McArdle's disease, Glycogen Storage Disease Type V) impairs muscle glycogen breakdown, leading to **exercise intolerance** and **muscle cramps** due to insufficient ATP production during exertion. - Patients often experience a "second wind" phenomenon where symptoms improve after resting, as free fatty acids become an alternative fuel source. *Hexokinase* - A deficiency in **hexokinase** would affect the first step of glycolysis, impacting glucose phosphorylation in all tissues, not specifically causing exercise-induced muscle cramps. - This deficiency is rare and typically presents with **hemolytic anemia** due to impaired erythrocyte metabolism. *Glucose-6-phosphatase* - A deficiency in **glucose-6-phosphatase** (Von Gierke's disease, Glycogen Storage Disease Type Ia) primarily affects the liver and kidneys, leading to **fasting hypoglycemia**, lactic acidosis, and hepatomegaly, not exercise intolerance. - Muscle glycogen metabolism is unaffected in this condition. *Hepatic glycogen phosphorylase* - A deficiency in **hepatic glycogen phosphorylase** (Hers' disease, Glycogen Storage Disease Type VI) mainly causes **hepatomegaly** and **mild hypoglycemia** because the liver cannot effectively mobilize its glycogen stores. - **Muscle glycogen metabolism** remains normal, so exercise intolerance and cramps are not characteristic symptoms.

Q: Classical galactosemia (Type I) is due to deficiency of which enzyme?

Galactose-1-phosphate uridyltransferase. ***Galactose-1-phosphate uridyltransferase*** - **Galactosemia Type I** (**classical galactosemia**) is caused by a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway of galactose metabolism. *Adenine phosphoribosyltransferase (APRT)* - Deficiency in **adenine phosphoribosyltransferase (APRT)** leads to **APRT deficiency**, characterized by **kidney stones** composed of 2,8-dihydroxyadenine. - This enzyme is involved in **purine salvage pathways**, not carbohydrate metabolism. *Fructose-1,6-bisphosphatase* - A deficiency in **fructose-1,6-bisphosphatase** causes **fructose-1,6-bisphosphatase deficiency**, a disorder of **gluconeogenesis**. - It results in **hypoglycemia** and **lactic acidosis**, especially during fasting. *Hexokinase* - **Hexokinase** phosphorylates glucose to **glucose-6-phosphate**, the first step in glycolysis. - Deficiency is rare but can lead to **nonspherocytic hemolytic anemia**.

Question 1

Which of the following is an aldose sugar?

Accepted Answer

Glyceraldehyde

Answer

Ribulose

Answer

Fructose

Answer

None of the options

Question 2

In the oxidative phase of pentose phosphate pathway, NADPH is produced in?

Accepted Answer

Cytosol

Answer

Mitochondria

Answer

Ribosome

Answer

Peroxisomes

Question 3

Which of the following enzymes participates exclusively in glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Phosphofructokinase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Hexokinase

Question 4

Which of the following is not a step in gluconeogenesis?

Accepted Answer

Conversion of pyruvate to acetyl-CoA

Answer

Conversion of glucose-6-phosphate to glucose

Answer

Conversion of oxaloacetate to phosphoenolpyruvate

Answer

Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate

Question 5

Which of the following enzymes do not catalyze an irreversible step in glycolysis?

Accepted Answer

Phosphoglycerate kinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Answer

Hexokinase

Question 6

Which enzyme converts glucose to sorbitol?

Accepted Answer

Aldose reductase

Answer

Sorbitol dehydrogenase

Answer

Aldolase B

Answer

Glucose-6-phosphate dehydrogenase

Question 7

Which of the following dietary fibers is most characteristically insoluble in water?

Accepted Answer

Lignin

Answer

Pectin

Answer

Hemicellulose

Answer

Cellulose

Question 8

When the insulin to glucagon ratio decreases, which enzyme is primarily active?

Accepted Answer

Glycogen phosphorylase

Answer

Glucose-6-phosphatase

Answer

Pyruvate carboxylase

Answer

Fructose 1,6-bisphosphatase

Question 9

An adolescent male patient presents to you with exercise intolerance. He gives a history of developing cramps on exertion. Which of the following enzyme deficiencies could be the cause?

Accepted Answer

Myophosphorylase

Answer

Hexokinase

Answer

Glucose-6-phosphatase

Answer

Hepatic glycogen phosphorylase

Question 10

Classical galactosemia (Type I) is due to deficiency of which enzyme?

Accepted Answer

Galactose-1-phosphate uridyltransferase

Answer

Fructose-1,6-bisphosphatase

Answer

Adenine phosphoribosyltransferase (APRT)

Answer

Hexokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?